There has conventionally been a semiconductor device including a HEMT (High electron mobility transistor) which is a field effect transistor having a heterojunction structure with GaN as a switching device. The HEMT is appropriate for higher-speed switching since GaN has a wide band gap, a high dielectric strength voltage, and a high saturation electron velocity.
For example, a HEMT using a heterojunction structure including aluminum gallium nitride (hereinafter referred to as AlGaN) and GaN is assumed to be a structure including a buffer layer and a heterojunction structure on a silicon substrate. The heterojunction structure is assumed to be a structure in which, e.g., an AlGaN layer is stacked on a GaN layer. The HEMT is made so that a gate electrode is formed on the AlGaN layer through a gate insulating film, and a source electrode and a drain electrode are formed on a top surface of the AlGaN layer on opposite sides of the gate electrode.
It has been proposed that, in a semiconductor device including such a HEMT, a gate insulating film is formed of a multi-layer film including, e.g., alumina (AlOx) and a silicon dioxide film (SiO2) (see Patent Literature 1). When such a structure is provided, interdiffusion of Si and Ga at the interface between the GaN layer and the gate insulating film is suppressed to allow an interface state to be suppressed. In addition, even when alumina is used, an energy band gap Eg is as large as that of SiO2. Accordingly, it is also possible to suppress a leakage current.
A structure has also been proposed in which a gate insulating film is formed of a multi-layer film including a layer which contains aluminum (Al), but does not contain nitrogen (N) and a layer which contains silicon (Si) and oxygen (O), and an upper layer is formed thicker than a lower layer. Additionally, a structure has also been proposed in which a gate insulating film is formed of a multi-layer film including aluminum oxide (Al2O3) and a silicon dioxide film and either one of the films is formed of a microwave plasma film.